A roadside bomb has exploded in northwest Pakistan killing two off-duty polio vaccinators and wounding their relative, officials said.

The blast occurred on Wednesday in the Safi town of Mohmand agency along the Afghan border where Taliban fighters are active.

The bomb went off near the house of polio vaccinator Mohammad Gul who was out walking with his brother, also a polio worker, and his cousin, although it was not clear whether they were specifically targeted.

A local security official also confirmed the bombing and casualties to the AFP news agency.

Nobody claimed responsibility for the bombing, but armed groups have been aggressively targeting immunisation workers across Pakistan.

The fighters allege polio vaccination is a cover for espionage or Western-conspiracy to sterilise Muslims.

Those conspiracy theories gained further traction after the CIA recruited a local doctor to start a vaccination programme during the hunt for Osama Bin Laden which dismayed many aid and health workers.

Some 60 people including health workers and police providing security have been killed in attacks on polio vaccination teams since December 2012.

The number of polio cases recorded in Pakistan passed 200 for the year last week, a 14-year-high.